Radio apparatus



March 15, 1932. s. E. ANDERSON RADIO APPARATUS Filed Oct. 24, 1924 m n wm June 15 Andaman.

Patented Mar. 15, 1932 UNITED STATES PATENT OFFICE SIDNEY 1!. ANDERSON,OF HAPLEWOOD, NEW JERSEY, ASSIGNOR TO WESTERN ELEC- IBIC COIPANY,INCORPORATED, OF NEW YORK, N. Y., A. CORPORATION OF YORK NEW

RADIO APPARATUS Application filed October 24, 1924. Serial No. 745,583.

This invention relates to radio receiving systems and more particularlyto methods of and apparatus for the tuning of receiving systems.-

5 Prior to the advent of radio broadcasting, stations were assigned wavelengths which varied by a given amount, and it was customary to considerthe wave length rather than frequency in tuning and allocation. Be-

lo cause of this tendency, some of the tuning units used at the radiostations were designed to have a linear wave length characteristic,which was advantageous in decreasing to some extent the tendency for theadjustment to become critical at the higher frequency positions.

With the advent of radio broadcasting, however, stations were allocatedon a basis of constant frequency difference which greatly increased thedifiiculty of selectively separating the waves transmitted bydifferentstations, es ecially at the higher frequencies employed or thispurpose.

With the recent widespread development of radio broadcasting, theimportance of the frequency difference between various transmitters hasbeen emphasized.

On the basis that a frequency band of 5,000 cycles is satisfactory forthe transmission of 80 high quality speech and music, the broadcastingstations have been uniformly spaced at frequency intervals of 10kilocycles. This means that with reference to wave length the stationsare much closer together at the high frequency end of the broadcastingrange than at the lower end of the broadcasting range, and hencefrequency discrimination between stations using short wave lengths isexceedingly difiicult.

11 general variable a1r condensers having semicircular plates are usedto control the selective circuits of broadcast receiving sets and, as aconsequence, the adjustment of the circuits becomes very critical as theposition of minimum capacity of the condenser is approached. This isbecause the increase in capacity is directly proportional to thecondenser setting over a major portion of the scale indicating therelative positions of the condenser plates, and a change in the settingof one division causes a much larger percentage change in the capacityof the condenser at the lower portion of the scale,

where the capacity approaches a maximum, than at the upper portion,where the capacity value is near a minimum.

The difficulty of frequency discrimination, between the waves radiatedby different broadcasting stations, can be overcome by using a tunedcircuit including a condenser adapted to vary the resonant frequency ofthe circuit in direct proportion to the condenser setting. F orinstance, if such a condenser were designed to cover the frequency rangefrom 500 to 1,500 kilocycles, and the scale were divided into 100divisions; the selective circuit of the receiver would be tuned to arelatively high frequency of say 1,350

.kilocycles at 15 divisions on the condenser scale and to the lowestfrequency in the broadcast range, namely 550 kilocycles at 95 divisionson the condenser scale.

The availability of condensers having the property mentioned above makespossible the solution of a problem for which there has heretofore beenno practical solution. The problem is that of adjusting two resonantcircuits simultaneously in such manner that throughout the completerange of adjustment, the two resonance frequencies differ by a. fixedconstant amount. This problem is encountered in the design of heterodynereceiving systems, and particularly, in the design of double detectionor super-heterodyne systeg'ls, when it is attempted to arrange fortuning by means of a single control. The frequency difference betweenthe received wave and that of the local oscillator is determined by theselective characteristic of the circuit traversed by the detected beatWave. In double-detection systems the beat frequency is usually from50,000 to 100,000 cycles per second, and the intermediate amplifyingapparatus usually includes a fixed circuit selective of this frequency.\Vith the designs heretofore available, the condensers used,respectively, for tuning the input receiving circuit and for tuning thelocal oscillator, could not be coupled together for operation by asingle control. The reason of the adjustments, were stronglydiscriminated against.

In accordance with the present invention a double-detection receivingsystem is adapted to be adjusted by a single control. The feature bymeans of which the result is secured comprises a tuning unit, theresonant frequency of which varies linearly, and by equal increments,for equal variations in the adjustments of its control element,throughout the operating range for which the unit is designed. Thetuning unit comprises a fixed inductance and a variable condenser,having its movable plate so designed that the resonance point of thetuned circuit may be varied as a linear function of the displacementthroughout the operating range of the unit.

An object of the invention is to accomplish the tuning of hetcrodynereceiving systems by means of a single control.

A second object is to permit the simultaneous adjustment of a pluralityof tuned circuits so that the resonance frequencies differ constantly byfixed amounts.

Another object of the invention is to cause the frequency to which anadjustabletuned circuit is resonant, herein called its resonantfrequency, to vary as a linear function of the position of theadjustable element over the operating range of the circuit.

A feature of the invention, essential to the achievements of the abovementioned objects, is an element, for use in a tuned circuit, whichcontrols the circuit so that its resonant frequency varies as a linearfunction of the position of the element throughout the operating rangeof the circuit.

In the specific embodiment of the invention, this element is a variableair condenser, the movable plates of which are so designed that theresonant frequency of the circuit wherein it is used will be a linearfunction of the angle of rotation of the plate.

Equations for determining the shape and area of the rotary plates of anadjustable condenser adapted to vary the selectivity of a tuned circuitindirectly with frequency and by equal amounts for equal adjustments ofthe condenser throughout the broadcasting range are given below.

The fundamental equation identifying the relation between the resonantfrequency and the angle of rotation in radians of the adjustable elementof a resonant circuit, where a linear relation between them is desiredis f=ab0 (1) where the frequency f is highest when the angle 0 0 and aand b are constants to be determined by the conditions to be met.

From the well known equation that where L represents the inductance,which is constant, and C identifies the capacity,

which is variable, in the circuit under consideration; we may write Theabove equation defines the law in accordance with which the enmeshedarea of the plates must vary in order that the linear relation betweenfrequency and angle of rotation may be secured. This equation isfundamental and is independent of the particular type of plate used orof the mode of adjustment. To develop the detail design of a condenser,it is necessary to determine an exact expression for the geometricalform that will produce the above described variation of the enmeshedarea.

7 The formulae for the design of a rotating condenser of conventionaltype will be developed. The rotating plates are in the form of segments,the radius of which measured from the axis of rotation varies with theangle of displacement. The fixed plates are so arranged that theenmeshed area of the moving plates as the angle of rotation is varied,comprises a variable sector limited on one side by the shortest radiusof the moving plates.

' If 1' represents the variable outside radius of the movable condenserplate and T is the radius of the segment which must be cut from thestationary plate in order to clear the shaft and spacers supporting therotary plates, we may write as the area. of a small sector:

From the previous equation relating the area to the angular rotation, asecond equation for the rate of change of the area may be formed,namely:

By combining the two equations for the rate of change of the area, therelationship between the radius r and the angle 0 is found, which givesthe desired law of capacity variation. This equation is The arbitraryconstants a, b, and k may be eliminated from Equation ('3) as follows:Let f., and far denote, respectively, the maximum and the minimumfrequencies corresponding to the extreme condenser settings, 180 or 1rradians apart, and let n be the ratio of f to far. Further, let mbe themaximum radius of the rotating blades, then from Equation 1) it followsthat Again by substituting the values of a and b in Equation (3) andsolving the equation for the value of 0 equal to 1r the value of k isfound to be Substituting the values thus found for a, b and k inEquation (3), the equation may be transformed to:

in which the arbitrary constants are now the two radii T and 1'11- andthe ratio n of the extreme frequencies.

A further simplification of the form of Equation (1) may be acomplishedby di viding both sides by 1'1 and denoting the ratios 61 and '1 by tand u, respectively; then If we let n=3, which is the ratio mostadaptable to the present broadcasting wavelength range with a reasonableoverlap, and

solve this equation for 7'1r=1", we may obtain a solution which isindependent of the exact dimensions of the plates.

The final calculations may be further simplified if we let s=18 t, i.e., if the equation is solved for 8=1, we obtain the radius of the plateat an angle of 10, or, if s= 18, the radius of the plate is equal to themaximum radius 1'. Making the substitutions we may write As a convenientfirst solution 1' was chosen as 2" and 13 as. This gives u=16/3, and ourequatlon becomes To determine the capacity of the condenser it isnecessary to determine the area of one plate. This may be computed bysubstituting the value of the radius 7" given by Equation (4) in theequation for the elemental sector area and integrating the resultingequation with respect to the angle 0 between the limits zero and 1;.tegration gives for the useful. area of one side of the plate the valueY For a plate having the dimensions given above the area in squareinches of the rotary plate will be For spacing of .025 between plates ofopposite polarity, constituting a single unit, the capacity thereofwould be 12.12 mmf. or with 60 units, corresponding to 30 movableplates, the capacity would be 727 mmf.

By using Equation (A), we can find the radius of the rotary plate of thecondenser for as many angles as desired. By plotting these differentradii as the function of the angle, the curve of the periphery of theplate may be obtained. By using Equation (B) we may obtain the area ofthe plate from which we may calculate the capacity of the condenser.

The novel features which are believed to be characteristic of thisinvention will be pointed out with particularity in the claims appendedhereto.. The invention itself, how- The result of such in- Figs. 4 and 5are circuits in which the tuning unit may be used to advantage,

Fig. 6 illustrates the application of the invention to adouble-detection receiving system for the purpose of providing a singletuning control.

In Fig. 1 is shown a simple tuned circuit consisting of inductance 15and capacity 16 typifying a circuit such as that employed in a wavetrap, wave meter, frequency meter or in a signaling system where a tunedcircuit is employed as a unit.

The advantages of a condenser, so designed that the resonant frequencyof the circuit is a linear function of the angle of rotation of themovable plates, will be apparent when it is noted that by using such acondenser the same precision and ease of adjustment is obtained over theentire frequency range.

In the condenser shown in Figs. 2 and 3, the stationary plates 1,together with spacing washers 2 and insulating end plates 6 and 7 areassembled on rods 3. The rods are secured at one end to plate 7 whilethe other ends, projecting through the plate 6, are threaded to receivenuts 4, whereby these elements may be clamped together to constitute aunitary structure. Electrical connection is made to the fixed plates bymeans of thumbnut 5.

Rotary plates8 separated by spacing washers 10 are mounted on shaft 9which is free to rotate in bearings provided in insulating end plates 6and 7 S ring 11 exerts pressure against the end of sha t 9 to producesufficient friction to prevent the weight of plates 8 from causingrotation and thereby varying the adjustment effected. Binding post 12 isconnected to the rotary plates through spring 11.

The condenser is shown mounted on an insulating panel 13 through whichshaft 9 extends. Knob 14 is mounted on shaft 9 for the purpose ofadjustment and has associated with it a dial for indicating the positionof the movable plate;

Fig. 4 shows a typical radio receiver consisting of an antenna coupledto the receiving circuit by a transformer 17 the secondary of which isconnected in parallel with a con denser 18 to constitute a circuitselective of the incoming high frequency wave.

The tuned circuit is included in the input circuit of a space dischargedevice 19, which operates as a detector and a signal indicating device20, herein shown as a telephone receiver, is included in the outputcircuit 0 the device 19.

By using a condenser 18 having thestraight line characteristic describedabove the difliculties attendant upon tuning the receiving circuit toselect the waves of different frequencies radiated from the variousbroadcasting stations will be avoided, since throughout the operatingrange of the receiv- Condensers 21 and 22 are used to control a theselective circuits associated with the first and second high frequencyamplifiers respectively.

The detector, D together with as many stages of low frequencyamplification as are desirable, supplies signal or audio frequencycurrent to the receiving device, herein shown as a telephone receiver.

By use of condensers 21 and 22 of the design described herein to controltuning of the selective circuits, a single control shaft may be employedto accurately adjust both tuned circuits to select any frequency lyingwithin the operating range for which the set is designed.

With other types of condensers heretofore proposed, it would bedifficult to maintain both circuits in resonance by varying the twocontrols an equal amount at the higher frequencies due to the criticaladjustment required, which magnifies slight mechanical variation due tomanufacturing processes.

Fig. 6 represents a conventional double detection system, consisting ofa loop antenna 23 which may be tuned to the frequency of the incomingwave by condenser 24.

Oscillator 25 produces a sustained wave the frequency of which isdetermined by condenser 26. These oscillations are combined with theincoming wave in detector 27 to produce an intermediate frequency wavewhich may be used to control an amplifier A, consisting of as manystages as are desirable.

The intermediate frequency wave is detected in the device D, to yieldthe signal current, which is supplied to the telephone or otherreceiving devices.

In the circuit just described, condensers 24 and 26 must at all times beadjusted to maintain constant the difference between the frequencyselected by the loo circuit and that of the local oscillations, which islargely determined by the tuned circuit associated with the oscillator.

With the types of condensers previously used, as pointed out above, itwould be impossible, while controlling the two condensers by a singleadjusting'means, to maintain the frequency difference constant.

However, if condensers 24 and 26 are built in accordance with thisinvention, it will be possible by mounting them upon the same shaft andangularly displacing one with respect to the other about the commonaxis, to operate them by a single control. Condensers so constructedwould be operative over 180 minus the angle of displacement.

This is a distinct advantage in receiving sets employing successivedetection.

In systems of the double detection type used at the present time, it isessential to adjust two separate controls, the settings of which atcertain, points in the operating range of the set are critical. Becauseof this fact these settings can only be accomplished with any degree ofaccuracy, by a skilled operator.

single control element can be used to effect a' lunch more accurateadjustment of the selective circuits than in the system now generallyused, and as a result there is provided a reuse by an unskilledoperator.

Although the invention has been shown as embodied in a particularstructure and particular circuits, it is to be understood that this :0invention is not limited thereto, but only in accordance with the spiritof the invention as defined in the following claims.

What isclaimed is: 1. In a radio receiving circuit containing aplurality of circuits tuned to different frequencies, variablecondensersadapted to control the resonant frequency of said circuits,said variable condensers including movable plates of such shape that theresonant fre- 3 quency of the tuned circuits varies as a straight linefunction of the angle of rotation of said plates, and a uni-controlmeans for said movable plates.

2. In a radio receiving system adapted to detect signals by beating thereceived waves with waves from a local oscillator, a tuned I circuit forthe received waves, a second tuned circuit for determining the frequencyof the local oscillations, variable elements in said tuned circuitsadapted for adjusting the resonance frequencies thereof, and means foradjusting the variable elements simultaneously by single control thefirst tuned circuit to any of a plurality of frequencies in a frequencyrange of the order of an octave, said elements being so proportioned anddisposed with respect to each other that the resonance frequencies ofsaid tuned circuits differ by a constant amount for all positions of thecommon control.

3. In a heterodyne receiving system, a

tuned circuit for received waves, a second With the arrangementdescribed above a ceiving set which is especially adapted for tunedcircuit for received waves, a second densers being proportioned toproduce linearvariations of the resonance frequencies of the respectivetuned circuits with respect to the degree of condenser adjustment, andbeing relatively displaced to provide a fixed difference between theresonance frequencies while the resonant frequency of one of said tunedcircuits is varied over a frequency range of the order of an octave.

5. A signal receiving system comprising an input circuit and anoscillator circuit, each of said circuits being provided with requisitetuning means including variable elements adapted to give a straight linefrequency variation, and said circuits being tuned to differentpredetermined frequencies, and means for simultaneously varying the saidtuning means so as to maintain constant said frequency difference.

6. A signal receiving system comprising an input circuit and anoscillator circuit, each of said circuits being provided with variabletuning condensers adapted to give a straight line frequency variation,said tuning condensers being relatively so adjusted as to give saidcircuits a predetermined frequency difference, and means to maintainconstant throughout the operating range said frequency difference.

7. A signal receiving system comprisin an input circuit and anoscillator circuit, eac of said circuits being provided with variabletuning means adapted to give a straight line frequency variation, thesaid tuning means being mechanically connected in fixed relation andrelatively adjusted to maintain a predetermined frequency differencebetween said circuits, and a tuning control common to the said tuningmeans.

8. .A signal receiving system comprising an input circuit and anoscillator circuit, each of said circuits being provided with variabletuning means adapted to give a straight line frequency variation, thevariable elements of the said tuning means being mounted upon a commonshaft and fixed thereon in such angular relation as to maintain apredetermined frequency diiference between said circuits.

9. In combination, a. tuned circuit, a second circuitituned to afrequency differing from a frequency to which the first circuit istuned, each of said circuits containing inductance and capacityelements, and movable control means for simultaneously varying thereactance of one of said elements of each tuned circuit, the variableelements being so proportioned that the frequency of the tuned circuitsvaries as a straight line function of the movement of said controlmeans, to maintain a constant difference between the frequencies towhich said circuits are tuned.

10. In combination, a plurality of circuits tuned to differentfrequencies each of said circuits containing a variable condenser forcontrolling the frequency to which the circuit is tuned, said variablecondensers having movable plates'of such shape that the resonantfrequency of the tuned circuits varies as a straight line function ofthe angle of rotation of said plates, and a shaft supporting saidmovable plates whereby the variable condensers are simultaneouslycontrolled.

11. In a heterodyne signal receiving system employing a super-audiblebeat frequency, an input circuit and an oscillator circuit, each of saidcircuits being provided with tuning means including variable elements,said circuits being tunable to different frequencies, the difference ofsaid frequencies being said beat frequency, and means for simultaneouslyvarying the said tuning means, said varying means so relativelyconnecting said tuning means and said variable elements as to maintainconstant said beat frequency. v

12. A superheterodyne receiver including a resonant circuit tunable to asignal frequency, an oscillator circuit tunable to a frequency whichdiffers from the signal frequency by a super-audible beatfrequency,

tuning means for each circuit including an element having a variableelectrical value, means for simultaneously varying said tuning means,the said varying means so relatively connecting said tuning means andsaid variable elements as to maintain said beat frequency a constant.

13. A method of operating a superheterodyne receiver which consists incollecting signal energy of a desired frequency by tuning a resonantcircuit to said frequency, producing local energy of a frequencydifi'ering from the signal frequency by 'a superaudible frequency bytuning an oscillation circuit, combining the local and si nal energiesto produce said difference requency, and simultaneously varying by equalmovements the tuning of the resonant and oscillation circuits to otherfrequencies, but maintaining the said super-audible difference frequencyconstant.

In witness whereof, I hereunto subscribe my name this 23d day ofOctober, A. D. 1924.

SIDNEY E. ANDERSON.

